The present invention relates to flexible sheet or roll stock film having elastic characteristics and to products formed of such film. Film-forming polymers are used to hot form the film with elastic and nonelastic or inelastic discrete polymer portions connected at a joint therebetween.
The films are useful in tapes, closure devices, labels and other constructions requiring a stretchable or elastic film element. The shear resistance and elastic property of the film in combination with the non-creep nature of the inelastic portions provide unique and valuable performance useful in a wide range of applications. The films may be used as a construction film for forming an element of a more comprehensive tape or closure system including industrial or craft applications requiring pieces to be secured together with tension during a dry cycle or medical applications such as medical tapes, suture tapes, nasal dilators, bandages and the like.
The films are particularly useful as substrates for closure systems such as mechanical and/or self-adhesive diaper fastening tapes or tabs. In such applications, the elastic characteristic of the film enhances fit, comfort, absorbency, containment and/or security of closure. The films may also be used as attachments to the waist or leg areas of a device or article of clothing for enhancing fit, comfort, and/or sealing characteristics. The film is especially useful in connection with disposable diaper tape fastening systems, and it is specifically described with respect to the same hereinafter.
Disposable diapers are known in the art and comprise multiple layer assemblies or laminates including an inner filling of absorbent fiber or material sandwiched between outer layers. One of the outer layers includes an absorbent material to be disposed against the user, and the other outer layer may comprise a waterproof plastic film for containment of waste within the diaper.
The use of stretchable fastening tapes or tabs in disposable diapers is disclosed in U.S. Pat. No. 3,800,796 to Jacobs. In Jacobs, a polymeric variation of the tape includes a semielastic strip having a fully extensible elastic central segment and two non-extensible inelastic terminal segments. The elastic segment comprises a heat-sealable elastomer such as a butadiene-styrene block polymer. The patent does not describe a coextrusion process for making film that may be used to form the tape.
U.S. Pat. No. 4,787,897 to Torimae et al. discloses a co-extruded diaper fastening tape with an elastic central segment and nonelastic terminal segments. The elastic segment includes triblock polymers containing 10 to 80 parts of a processing aid comprising a hydrogenated terpene resin or alicyclic hydrocarbon having a melt or softening point of 80xc2x0 C. and a molecular weight of 400 to 2000.
It has now been discovered that certain polymeric materials comprising thermoplastic elastomers may be used to form reversibly extensible sheets or roll film material. The sheets or films include discrete portions of thermoplastic elastomer having elastic characteristics joined with portions of thermoplastic polymer having nonelastic properties.
The thermoplastic elastomers and the thermoplastic polymers are film-forming polymers that may be hot formed as by simultaneous coextrusion. For example, the film-forming polymers may be co-extruded in a pattern of laterally spaced and longitudinally parallel polymer lanes or stripes extending in the machine direction with integrally formed joints adhering adjacent lanes together.
The film material according to the invention has excellent extrusion characteristics and superior joint strength between the elastic and the nonelastic polymers. Also, the film materials have improved elevated temperature shear properties and improved caliper and roll conformation as indicated by wound rolls of near cylindrical configuration.
The thermoplastic polymer may be a polyolefin such as polyethylene, polypropylene, poly(ethylene-propylene), poly(ethylene-vinyl acetate), poly(styrene-butadiene), or copolymers or blends thereof. A minor amount of an ethylene-propylene component may be incorporated in the thermoplastic polymer. Polypropylene is a preferred thermoplastic polymer. Films of such polymers, e.g. 0.1 to 20 mils thick, exhibit little or no recovery from stretching or deformation and exemplify the inelastic or nonelastic properties of interest herein.
Thermoplastic elastomers of interest herein are block copolymers having or containing the tetrablock structure A-B-A-D, the triblock structure A-B-A and, optionally, the diblock structure A-B in lesser amounts as a minor component. In such block structures, A represents a block which is non-rubbery or glassy or crystalline at service temperature, e.g. about 100xc2x0 F. in the case of diaper tapes, and B and D, which may be the same, each represent a block which is rubbery or elastomeric at service temperature. At elevated temperatures, the A, B and D blocks are sufficiently fluid to enable coextrusion of the thermoplastic elastomer. Films of such polymers, e.g. 0.1 to 20 mils thick, exhibit recovery from stretching or deformation below their yield point and exemplify the elastic properties of interest herein.
The thermoplastic elastomer may be blended with a thermoplastic polymer end block reinforcing agent of relatively high molecular weight, e.g. a molecular weight greater than 2000 and, more preferably, in the range of 3000 to 5000, and higher. Such reinforcing agents improve elevated temperature shear strength. Most unexpectedly, such reinforcing agents have been found herein to increase the caliper or gauge uniformity of wide sheet coextrusions, e.g. 1500 mm or 60 inches.
Preferred reinforcing agents are high molecular weight aromatic compounds such as polyphenylene oxide (or polyphenyl ether) which have molecular weights in the range of 3,000 to 30,000 and higher. These reinforcing agents are added in amounts ranging from 3 to 9, and more preferably, from 8 to 9 parts by weight based on the weight of the elastomer component. Other preferred reinforcing agents comprise pure monomer resins of polycyclic arenes including substituted and unsubstituted vinylarenes such as styrene and methylstyrene. These are used in amounts ranging from 5 to 25, and more preferably, from 12 to 14 parts by weight based on the weight of the elastomer component.
Other conventional additives such as antioxidants, colorants and processing aids may be added to either or both the elastic and nonelastic polymers.
A diaper fastening tape should have the ability to withstand an applied load of 500 grams for more than 1,000 minutes at 100xc2x0 F. (100xc2x0 F. being used to simulate body temperature.) Some of the films in accordance with the invention have shear strengths that exceed 10,000 minutes at 100xc2x0 F. under a 500 g load.
The reason for the improved shear strength is believed to be related to the unique nature of the joint at the joining plane or interface where the thermoplastic meets the tetrablock thermoplastic elastomer. The strength of the joint is believed to be improved by the unique interaction of the saturated tetrablock elastomer with the thermoplastic polymer. More particularly, preferred tetrablock elastomers include a terminal segment or free tail block of ethylene-propylene that provides a hydrocarbon structure similar to the hydrocarbon groups of the thermoplastic so as to favor interaction and improved joint strength. This effect may be further enhanced by addition to the thermoplastic of additional similar groups such as ethylene propylene rubber (EPR) which is used as an impact modifier. In contrast, elastic films made with conventional diblock and/or triblock thermoplastic elastomers do not develop such good joint strength.
Shear strength is improved also by the use of increased numbers of elastomer lanes of reduced size. For example, the shear strength of films having a 2xe2x80x3 wide polyolefin lane and a xc2xcxe2x80x3 wide rubber lane were greatly improved by reducing the lane widths to about xc2xdxe2x80x3 and {fraction (1/16)}xe2x80x3 to xe2x85x9xe2x80x3. Generally, rubber lanes that are of such narrower dimension give much higher shear times. For example, a tape having a single rubber lane may give a time of 1,000-2000 minutes. A tape of the same material but with multiple more narrow lanes of rubber will yield upwards of 10,000 to 20,000 minutes. Generally, the elastomer lane width in a 3 to 8 mil thick film should be in the range of from about {fraction (1/16)}xe2x80x3 to xe2x85x9xe2x80x3.
The foregoing improvement apparently relates to the division of work of elasticity into two or possibly more lanes. This is believed to be related to the facts that the propagation of a fracture due to the presence of a cut or nick is pulling rate dependent and that stresses within an elastomeric lane that is failing are focused on a very small portion of the elastomer. If there is only one elastomeric lane in the stretch zone of a tab that is being elongated, and that one lane has a nick or cut, the stresses associated with said elongation are very quickly focused on the point of crack propagation and complete failure occurs in a short time. If there is a multiplicity of elastomeric lanes, only one of which has a cut or nick, the propagation of a crack across the lane with the cut or nick is delayed because there are other, defect-free elastomeric lanes within the stretch portion of the tab that can accommodate the elongation without suffering failure. Ultimately, of course, as elongation or pulling rate increases to very high values, a tab will suffer failure, but at ordinary values the elastic response time of the defect-free lanes is short enough to substantially delay failure in the lane that has a defect.
The end block reinforcing agents herein are of relatively high molecular weight, i.e. greater than 2000, as compared with prior art, and they have been found also to contribute to the improved caliper and roll conformation. Heretofore, coextrusion of adjacent elastic and nonelastic polymer materials through a single die opening resulted in films having sufficiently different thicknesses in the different polymer areas to cause nonuniform and unacceptable roll conformation. This resulted in limited film widths that are commercially cost ineffective. For example, coextruded films in accordance with U.S. Pat. No. 4,787,897, supra, were found to have undesirable caliper variation in the width direction yielding unacceptable roll conformation. That is, the rolls are nonuniform and have varying diameter dimensions along the axis of the roll and, for example, may have gauge bands of increased thickness around its circumference at the locations of the elastomeric lanes. The gauge bands cause the elastomer to be distorted, which in turn, causes the film to bunch-up and develop wrinkles upon unwinding, processing and re-winding the roll. The poor roll conformation significantly limits the roll width, e.g., to that of one or two diaper tape widths, which is not acceptable for commercial manufacture.
The arrangement and sizes of the elastomer and nonelastomer polymer areas have been found also to provide improvements in roll conformation. Generally, improvements in roll conformation have been achieved by limiting the width of the elastomer portion. Also, improved roll conformation is achieved by providing increasing numbers of elastomer and nonelastomer portions of reduced size; i.e., an increased number of laterally more narrow lanes. Roll conformation is improved also with overall uniformity in the elastomer and nonelastomer arrangement across the entire width of the sheet.
The film optionally includes one or two thin skins of polymer that completely cover the adjacent lanes. That is, the distinct elastic and nonelastic polymer areas form substantially the entire thickness of the film, but a substantially thinner skin may be formed along one or both of the film surfaces. Thermoplastic skins are preferred since they will generally have less surface friction and will aid machine processing as compared with skins formed of thermoplastic elastomer. Also, thermoplastic skins will reduce, if not eliminate, the tendency of the film to block as by adherence of contacting elastomer lanes or polymer areas.
The thin film lowers the coefficient of friction on that side of the film on which it is present to provide handling advantages in web processing. The skin also provides a non-elastomeric surface on which to coat an adhesive, and this may be of particular advantage if the adhesive or other coating is not inherently stable over time when coated directly onto the surface of thermoplastic elastomers.